Wavelength selective optical switch

ABSTRACT

A wavelength selective switch, in which an input optical signal is wavelength-dispersed and polarization-split in two angularly oriented planes. A polarization rotation device, such as a liquid crystal polarization modulator, pixelated along the wave-length dispersive direction such that each pixel operates on a separate wavelength channel, rotates the polarization of the light signal passing through the pixel, according to the control voltage applied to that pixel. The polarization modulated signals are then wave-length-recombined and polarization-recombined by means of similar dispersion and polarization combining components as were used to respectively disperse and split the input signals. The direction of the output signal is determined by whether the polarization of a particular wavelength channel was rotated by the polarization modulator pixel, or not. In this way, a fast, wavelength dependent, optical switch is provided, capable of use in WDM switching applications.

FIELD OF THE INVENTION

The present invention relates to the field of devices for the switchingof optical signals, according to the wavelength of the light bearing theinformation, by means of spatially selective polarization rotation ofthe light, especially for use in optical communication networks.

BACKGROUND OF THE INVENTION

Fast all-optical switching of information is an essential element ofmodem optical communication systems, enabling the slower electronicfunctions to be reserved only for the input and output terminals of thenetwork, where the speeds required are those of individual channels, andnot of the network throughput. In WDM systems, the information issegregated with regard to its source and destination according to thewavelength of the particular optical signal being transferred, and aswitch for use in such a system must therefore be able to route eachsignal automatically according to its wavelength.

There exist a number of prior art devices for performing this function,such as that described in U.S. Pat. No. 5,414,540 to J. S. Patel et al.,for “Frequency-selective optical switch employing a frequency dispersiveelement, polarization dispersive element and polarization modulatingelements”, hereby incorporated by reference in its entirety, and thosedevices mentioned in the references cited therein. However, most ofthose devices have one or more disadvantages in that they are eithercomplicated to build, or to align, or utilize expensive component parts.For instance, the complete embodiment of the switch described in U.S.Pat. No. 5,414,540 contains on its input side, as illustrated in FIG. 11of the patent, a wavelength dispersive element (not shown), apolarization dispersive element to displace the beam polarizations, suchas a birefringent crystal, a half-wave plate element, a focusingelement, another polarization dispersive element such as anotherbirefringent crystal, and a segmented liquid crystal polarizationmodulator. The same number of components are required on the output sidealso. In the embodiment shown in FIGS. 1-4, the input elements include apolarization alignment component (not shown) to provide a specificpolarization direction to the input beam, a wavelength dispersiveelement, a focussing element, a polarization displacement element suchas a birefringent crystal, and a segmented liquid crystal polarizationmodulator. In addition, the same components are required on the outputside also. Such a switch, in any of its embodiments, therefore containsa large number of components and is thus complicated to construct.

There therefore exists a need for a simple wavelength-selective opticalswitch, employing a smaller number of separate components than that ofcommonly available current devices.

The disclosures of each of the publications mentioned in this sectionand in other sections of the specification, are hereby incorporated byreference, each in its entirety.

SUMMARY OF THE INVENTION

The present invention seeks to provide a new wavelength selectiveoptical switching device, which is simple in construction, and uses asmall number of component parts, thus overcoming some of thedisadvantages of previously available switches.

There is thus provided in accordance with a preferred embodiment of thepresent invention, a wavelength selective switch, wherein an inputoptical signal is spatially wavelength-dispersed and polarization-splitin two angularly oriented planes, preferably perpendicular, planes. Thewavelength dispersion is preferably performed by a diffraction grating,and the polarization-splitting by a polarizing beam splitter. Apolarization rotation device, such as a liquid crystal polarizationmodulator, pixelated along the wavelength dispersive direction such thateach pixel operates on a separate wavelength channel, is operative torotate the polarization of the light signal passing through each pixel,according to the control voltage applied to the pixel. The polarizationmodulated signals are then wavelength-recombined andpolarization-recombined by means of similar dispersion and polarizationcombining components as were used to respectively disperse and split theinput signals. The direction of the resulting signal output isdetermined by whether the polarization of the particular wavelengthchannel was rotated by the polarization modulator pixel, or not. In thisway, a fast, wavelength dependent, optical switch is provided, capableof use in WDM switching applications. According to a second preferredembodiment, the use of a reflecting surface at the plane of symmetry ofthe switch, after the polarization modulator, enables the number ofcomponents in the switch to be substantially reduced, to almost halfthat of the first embodiment.

There is further provided in accordance with another preferredembodiment of the present invention a wavelength dependent switch,comprising a polarization splitting element receiving an input beamincluding a plurality of wavelengths, and operative to spatially dividethe input beam into separate polarization components, a wavelengthdispersive element receiving the polarization-divided components of theinput beam, and operative to spatially disperse each of thepolarization-divided components of the beam into its wavelengthcomponents in a plane disposed at an angle to the plane in which thepolarization components are divided, a polarization modulating element,pixelated along the direction of the wavelength dispersion such thateach pixel is associated with a separate wavelength, each pixel of thepolarization modulating element being operative to rotate the directionof the polarization of a beam passing through the pixel according to acontrol signal applied to the pixel, and a reflecting surface operativeto reflect the beam after polarization modulation back through thewavelength dispersive element and the polarization splitting element,such that the beam outputs in a direction according to the controlsignal applied to the pixel. The polarization modulating element maypreferably be a liquid crystal element.

According to yet another preferred embodiment of the present invention,there is further provided a wavelength dependent switch, including afirst polarization splitting element receiving an input beam including aplurality of wavelengths, and operative to spatially divide the inputbeam into separate polarization components, a first wavelengthdispersive element receiving the polarization-divided components of theinput beam, and operative to spatially disperse each of thepolarization-divided components of the beam into its wavelengthcomponents in a plane disposed at an angle to the plane in which thepolarization components are divided, a polarization modulating element,pixelated along the direction of the wavelength dispersion such thateach pixel is associated with a separate wavelength, each pixel of thepolarization modulating element being operative to rotate the directionof the polarization of a beam passing through the pixel according to acontrol signal applied to the pixel, a second wavelength dispersiveelement receiving the polarization-modulated components of the beam, andoperative to combine the wavelength components after passing throughtheir wavelength associated pixels, into a single multi-channel beam,and a polarization combining element receiving the separate polarizationcomponents of the single multi-channel beam, and operative to combinethe polarization components into a single output beam, each wavelengthcomponent of the output beam outputting in a direction according to thecontrol signal applied to the pixel associated with the wavelength. Thepolarization modulating element may preferably be a liquid crystalelement.

In accordance with yet another preferred embodiment of the presentinvention, there is provided a wavelength dependent switch, comprisingsequentially:

-   (i) a polarization beam splitter having a first and second port, for    receiving an input beam comprising at least two wavelength    components, and operative to spatially split the input beam into    beams of separate polarization components,-   (ii) a dispersive element receiving the beams of separate    polarization components, and operative to spatially disperse the    wavelength components of each of the beams in a dispersion plane    disposed at an angle to the plane in which the polarization    components are split,-   (iii) a polarization modulating element, pixelated along the    direction of the dispersion such that each pixel is associated with    a separate wavelength component, each pixel of the polarization    modulating element being operative to rotate the direction of the    polarization of light passing through the pixel according to a    control signal applied to the pixel, and-   (iv) a reflecting surface operative to reflect the light after    polarization modulation, back through the dispersive element and the    polarization beam splitter, such that each wavelength component of    the light is directed to one of the two ports according to the    control signal applied to the pixel associated with the wavelength.

In the above-described switch, the polarization modulating element maypreferably be a liquid crystal element. Furthermore, the angle may besuch that the dispersion plane is essentially orthogonal to the plane inwhich the polarization components are split. Additionally andpreferably, the separate polarization components may be two orthogonalcomponents.

There is further provided in accordance with yet another preferredembodiment of the present invention, a switch as described above, andalso comprising circulators at each of the first and second ports, suchthat the switch is operative to switch a wavelength component of asignal input through either of the circulators such that it outputseither of the circulators according to the control signal applied to thepixel associated with the wavelength.

In accordance with still another preferred embodiment of the presentinvention, there is provided a wavelength dependent switch, comprising:

-   (i) a polarization beam splitter having a first input port, for    receiving an input beam having at least two wavelength components,    and operative to spatially split the input beam into beams of    separate polarization components,-   (ii) a first dispersive element receiving the beams of separate    polarization components, and operative to spatially disperse the    wavelength components of each of the beams in a dispersion plane    disposed at an angle to the plane in which the polarization    components are split,-   (iii) a polarization modulating element, pixelated along the    direction of the dispersion such that each pixel is associated with    a separate wavelength component, each pixel of the polarization    modulating element being operative to rotate the direction of the    polarization of light passing through the pixel according to a    control signal applied to the pixel,-   (iv) a second dispersive element receiving the light, and operative    to combine the separate wavelength components of each of the beams    into multi-wavelength beams, and-   (v) a polarization beam combiner having two output ports, for    receiving each of the multi-wavelength beams, and operative to    combine the polarization components such that each wavelength    component is directed to one of the two output ports according to    the control signal applied to the pixel associated with the    wavelength.

In the previously-described switch, the polarization modulating elementmay preferably be a liquid crystal element. Furthermore, the angle maybe such that the dispersion plane is essentially orthogonal to the planein which the polarization components are split. Additionally andpreferably, the separate polarization components may be two orthogonalcomponents.

There is further provided in accordance with still another preferredembodiment of the present invention, a wavelength dependent switch asdescribed above, and also comprising a second input port disposedessentially orthogonal to the first input port, such that the switch isoperative to switch a wavelength component of a signal input to eitherof the input ports, to either of the output ports, according to thecontrol signal applied to the pixel associated with the wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description, taken in conjunction with thedrawings in which:

FIGS. 1A and 1B schematically illustrate different views of a wavelengthselective optical switch, constructed and operative according to a firstpreferred embodiment of the present invention. FIG. 1A is a view of thedevice from the polarization splitting plane, while FIG. 1B is a view ofthe same device from the wavelength dispersion plane, orthogonal to thepolarization splitting plane;

FIGS. 1C and 1D are schematic illustrations of simplifying adaptationsof the 2×2 switch shown in FIGS. 1A and 1B, enabling its use as a 1×2switch, or as a 2×1 switch when used in the reverse direction;

FIG. 1E illustrates schematically yet another embodiment of a wavelengthselective optical switch, according to a further preferred embodiment ofthe present invention, in which polarization dependent loss iscompensated for either position of the switch;

FIGS. 2A and 2B schematically illustrate a wavelength selective opticalswitch, constructed and operative according to a further preferredembodiment of the present invention, using a reflective surface toreduce the size and complexity of the switch;

FIG. 3 schematically illustrates the use of dual fiber collimators asinput/output devices in the embodiment shown in FIGS. 2A and 2B;

FIG. 4A is a schematic illustration of a stacked, multiple channel,wavelength selective switch, according to yet another preferredembodiment of the present invention, using common dispersive elements;FIG. 4B is a schematic illustration of a multiply parallel wavelengthselective switch with multiple inputs and outputs, in which the multipleinput and output fibers use the same optical elements;

FIG. 5 is a schematic illustration of a stacked, multiple channel,wavelength selective switch, according to yet another preferredembodiment of the present invention, similar to that shown in FIG. 4A,but using also common focussing lenses and a common liquid crystalelement;

FIG. 6 is a schematic illustration of a stacked, multiple channel,reflective wavelength selective switch, according to yet anotherpreferred embodiment of the present invention, using a commondiffraction grating, focussing lens and liquid crystal element; and

FIG. 7 is a schematic illustration of a reflective wavelength selectiveswitch, similar to that shown in FIG. 6, but using also a commonfocussing lens for both channels.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference is now made to FIGS. 1A and 1B, which illustrate schematicallydifferent views of a wavelength selective optical switch, constructedand operative according to a first preferred embodiment of the presentinvention. FIG. 1A is a view of the device from one plane, known as thepolarization splitting plane, while FIG. 1B is a view of the same deviceas seen from a plane preferably orthogonal to the first and known as thewavelength dispersion plane. The operation of the wavelength selectiveoptical switch can be understood by reference simultaneously to thesignal paths shown in FIGS. 1A and 1B.

An input optical signal 10, such as would be obtained after collimationfrom the end of an optical fiber 12, is input into a polarization beamsplitter (PBS) 14, shown in FIG. 1A as a split prism PBS, though it isto be understood that other preferred types of PBS may also be used forthis function. The PBS is so orientated that it preferably splits theinput signal into its two orthogonal polarization directions, markedP_(y) and P_(z), where P_(y) is in the plane of the paper, and P_(z) isout of the plane of the paper in the embodiment shown. The position ofthe axes x, y, and z are defined in FIG. 1A. The two polarizationcomponents of the signal, also commonly known as the p and the scomponents, are incident on a dispersive element, such as a diffractiongrating 16 in the preferred embodiment shown. The grating is operativeto disperse the different wavelengths λ₁, λ₂, λ₃, . . . λ_(n) intodifferent directions, according to their wavelength. The grating isaligned such that the wavelength dispersion direction, shown in theplane of FIG. 1B, is preferably perpendicular to the polarizationsplitting direction, shown in the plane of FIG. 1A. The optical signalis thus now wavelength-dispersed and polarization-split in two differentplanes, preferably orthogonal to each other. Though the gratings in theembodiment shown in FIGS. 1A and 1B are transmissive gratings, it isunderstood by one of skill in the art that the gratings, either one orboth, could equally be reflective gratings. Similarly, in the embodimentshown in FIGS. 2A and 2B hereinbelow, the single grating could be areflective grating.

The dispersed components of the signal are then imaged by means of alens 18 located at a distance equal to its effective focal length fromthe diffraction grating onto a pixelated polarization rotation element,20, such as a pixelated liquid crystal device, which is divided up intopixels, 24, 26, 28 . . . one for each wavelength channel to be directedby the switch. The light from each dispersed wavelength range is imagedonto a separate pixel of the device, as is seen in FIG. 1B. For the sakeof clarity, only two dispersed wavelengths are shown in FIG. 1B, but itis to be understood that there can be as many wavelength channels asthere are pixels. FIG. 1A shows a view of the switch from the side, andhence only pixel 24 is shown, carrying the signals of wavelength λ₁.Components of the signal having polarization in the P_(z) direction andin the P_(y) direction both pass through pixel 24, but laterallydisplaced from each other. The pixels are switched by means of controlvoltages V applied through an array of transparent electrodes on theliquid crystal surfaces, as is known in the art of passive matrix liquidcrystal arrays. Alternatively and preferably, the liquid crystal arraymay be an active matrix type, with individual thin film transistorsproviding the drive current for each individual pixel.

After passing through the liquid crystal device 20, the light signalsare imaged, preferably by means of a second focusing lens 30, ontoanother dispersive grating 32, similar in characteristics to the firstgrating 16. According to a preferred embodiment of the presentinvention, the polarization rotation element is preferably located atthe back focal plane of the focusing lens 18, and at the front focalplane of the lens 30, such that the overall assembly has a 4-fconfiguration, for optimum optical performance. Grating 32 is operativeto recombine the different wavelengths λ₁, λ₂, λ₃, . . . λ_(n) comingfrom their respective wavelength-dispersed directions, into a singlebeam path 34 in the wavelength-dispersed plane, though still spatiallysplit into its two polarization components in the polarization-displacedplane. These polarization components are then input to a polarizationbeam combiner (PBC) 36, where the polarization components are recombinedto form an output signal 42, which can be collimated and input into afiber 44 for onward transmission after switching.

The switch is operated by activation of the pixels of the liquid crystaldevice. Application of the required control voltage V to a pixel causesthe polarization passing therethrough to rotate, while inactivation ofthe liquid crystal pixel allows the signal to pass through with itspolarization unchanged. Thus, as is shown in the preferred embodiment ofFIG. 1A, when, the relevant pixel 24 of the liquid crystal device 20 forwavelength channel λ₁ is not activated, the polarizations P_(y) andP_(z), are unrotated, and the reconstituted signal outputs from thepolarization beam combiner 36 into fiber 44. For reasons of clarity, theoutput fiber 44 is not shown in FIG. 1B, as it is located out of theplane of the drawing. It should be noted that the output beam 42 isdirected orthogonally to that of the input beam 10, even though thepolarizations of the two components of the beam have not been changed bythe liquid crystal. This is a result of the focusing of the beams bymeans of two lenses through the liquid crystal device 20, causing thetwo laterally displaced polarization components of the beam to crossover and thus to change their mutual positions, such that the twocomponents behave as if their polarizations had been rotated by theliquid crystal.

If, however the pixel 24 of the liquid crystal device 20 is activated insuch a manner as to introduce a phase change of π for bothpolarizations, the polarizations P_(y) and P_(z), are rotated such thatthe polarization P_(y) becomes P_(z) and the polarization P_(z) becomesP_(y). When recombined in the polarization beam combiner 36, and takinginto account the beam crossover because of the imaging lenses, theresultant signal exits in the direction 38 parallel to the entrydirection, and inputs a second output fiber 40.

The above-described optical arrangement therefore behaves as a 1×2optical switch device which can direct an input signal 10 of wavelengthλ₁ into one of two output fibers 40, 44, by means of switching a liquidcrystal pixel 24. It is understood that by reversing the direction ofoperation of the switch, it operates as a 2×1 switch, with inputs oneither of fibers 40 and 44 being directed to the input fiber 12according to the setting of the liquid crystal pixels.

Each wavelength channel shown in FIG. 1B has its own liquid crystalpixel, the switching of which causes the light of that wavelength toinput into one or other of the output fibers. The switch is thus able todirect wavelength separated packets of optical information intodifferent paths, according to their wavelengths, by means of a switchingroutine of control voltages applied to the various wavelength designatedpixels of the liquid crystal device.

It should be noted that the novel construction and operation of theswitch of the present invention makes it essentially polarizationindependent, besides any residual polarization dependent loss whichthere may be in the grating or in the PBS. This feature is important foruse in fiber optical systems, since, as is known, the polarization of asignal transmitted down an optical fiber is generally randomized. Thereason for this polarization independence is that at the input 10 to thePBS, independently of the polarization direction of the input signal,any input signal can be split into two orthogonal components havingpolarization directions parallel to P_(y) and P_(z) relative to theorientation of the PBS, and each component is separately switched or notswitched to output 42 or 38, according to the state of the liquidcrystal pixel for that wavelength. On recombination of these twopolarization components, the original signal is regained. It isunderstood that if the polarization direction of the input signalhappens by chance to be exactly parallel to the P_(y) or the P_(z)direction, then light with only one component of polarization traversesthe switch.

According to another preferred embodiment of the present invention, asecond input fiber 50 is disposed at the orthogonal input port of thepolarization beam splitter 14. The input signal 48 from the second inputfiber 50, after passing through the polarization beam splitter 14, haspolarizations reversed from those shown in FIG. 1A. Thus, the upper pathshown in FIG. 1A has P_(y) polarization direction, while the lower pathhas a P_(z) polarization. Therefore, on passage through the liquidcrystal device, oppositely to the effect on the input signal from fiber12, activation of a pixel sends the input signal from fiber 50 out tofiber 44, and non-activation directs it to fiber 40. This preferredembodiment is therefore operative as a 2×2 optical switching network.Larger switching networks can be constructed by cascading such switches.As is evident from all of the above description, it is to be understoodthat by using only part of the capabilities of the switch of the presentinvention, 2×1, 1×2, or even 1×1 switches can also be implemented, thelast mentioned being useful as a channel blocker.

According to a further preferred embodiment, a half wave plate 22 can beinserted close to the liquid crystal device 24 in order to minimize thepolarization dependent loss (PDL). Such a half wave plate rotatesthrough 90° the polarization of light passing through it, such that inthe example shown in FIG. 1A, polarization P_(y) is converted to P_(z)and vice versa. Therefore, any difference in polarization dependent losssuffered by the incoming light during transit through the left hand sideof the switch system, i.e. before impinging on the half wave plate, willbe compensated during passage in the right hand side of the switchsystem, since the polarization directions of the orthogonally polarizedcomponents are interchanged and the switch is approximately right-leftsymmetrical. It should be noted that since the polarization directionsare reversed after passage through the half-wave plate the output for anunactivated pixel becomes port 40 and for an activated pixel, port 44.Alternatively and preferably, any other polarization rotating elementwhich rotates the polarization direction by 180°, such as a Faradayrotator, can be used instead of a half wave plate for this purpose.

Reference is now made to FIGS. 1C and 1D, which schematically illustratewavelength selective optical switches, constructed and operativeaccording to two further preferred embodiments of the present invention.The switches shown are simplified embodiments of those shown in FIGS. 1Aand 1B, for use as a 1×2 or as a 2×1 switch. In the embodiment shown inFIG. 1C, the multi-wavelength input signal 10 is applied through asingle input fiber 12, and is split into its orthogonal polarizationcomponents P_(y) and P_(z) by means of the input PBS 14. A half waveplate 15 is located at one output of the PBS, and is operative, in theembodiment shown, to rotate the P_(y) component into the P_(z)direction. Both of the polarization-split channels thus now have thesame polarization direction, P_(z) in the embodiment shown. Thesecomponents are then both passed through the dispersion grating 16, wherethey are wavelength dispersed in the dispersion plane (not shown) asshown in the embodiment of FIGS. 1A and 1B. However, unlike thatembodiment, both signals now have the same polarization direction, andthis enables the use of a high efficiency grating 16, thereby providingthe switching array with a significantly lower insertion loss than thatshown in the previous embodiment of FIGS. 1A and 1B. After thepolarization rotating device, which is operative in determining whichwavelength signal has its polarization switched and which not, and thewavelength combining grating 32, one of the polarization-split channelshas a farther half wave plate 33 in its path, operative in thisembodiment to rotate the P_(z) component back into a P_(y) component, sothat the output PBS 36 can direct each signal to its determined output,along path 38 or 42, depending on the state of the relevant pixel of theliquid crystal device. It should be noted that since the polarization ofthe light in both polarization dispersion planes of the switch isidentical, there is no need for a half wave plate 22 to compensate forpolarization dependent losses, as was shown in the embodiments of FIGS.1A and 1B.

The embodiment shown in FIG. 1D is similar to that shown in FIG. 1C, inthat the switch is constructed such that the liquid crystal polarizationrotating element operates on parallel polarization signals. However, inthe embodiment of FIG. 1D, the input signal 12 is split into itsorthogonal polarization components, one of which is rotated to bringboth components parallel, preferably by means of a birefringent crystalsuch as YVO₄ 13 with a half wave plate 17 on one half of its outputport, as is known in the art. Such a component, in an integrated form,is available commercially from several companies, including JDSU-CasixCorp., of Fuzhou, China, and is known by Casix as a C-polarizer. Theresult is the generation of two spatially-displaced, parallel-polarizedcomponents of the input signal, similar to those generated in theembodiment of FIG. 1C, but using a simpler and more compact opticalarrangement using less components. The remainder of the 1×2 switch ofthis preferred embodiment is identical to that described in relation tothe embodiment of FIG. 1C.

Reference is now made to FIG. 1E, which illustrates schematically yetanother embodiment of a wavelength selective optical switch, accordingto a further preferred embodiment of the present invention. Theembodiment shown in FIG. 1E overcomes a drawback in the use of a singlehalf wave plate 22 close to the liquid crystal element to eliminatepolarization dependent losses (PDL), as shown in the embodiment of FIG.1A. In the scheme of FIG. 1A, PDL is effectively compensated for so longas the liquid crystal is unswitched. As soon as the operative pixel ofthe liquid crystal is switched, another 180° phase shift is introducedinto the optical path, thereby nullifying the effect of the 180° phaseshift introduced by the half wave plate. The PDL is therefore no longercompensated when the switch is activated.

In the switch of FIG. 1E, a small half wave plate 41 is inserted intothe path of the upper polarization-split beam exiting the input PBS 14,such that in the example shown, its polarization is rotated from P_(z)to P_(y). Both polarization dispersion channels then have the samepolarization direction, P_(y), and therefore traverse the switch pathwithout engendering essentially different levels of PDL. At the outputPBS 36, a similar half wave plate 43 is inserted into the same channelas that having the half wave plate at the input, thereby reverting thepolarization direction back to P_(z), for outputting as in the originalscheme of FIG. 1A. Since both beams traverse the liquid crystal with thesame polarization direction, and without any additional half wave platein its vicinity, switching of the liquid crystal pixel has no effect onthe PDL compensation of the channels. The only difference intransmission between the two liquid crystal states is that engendered bythe difference in grating efficiency for the two polarization states,since the transmission efficiency of the input through fiber 12 isrelated to the P_(z) insertion loss at both gratings, while that offiber 50 is related to the P_(y) insertion loss for both gratings. Thisdifference is much smaller than the PDL differences.

It should be mentioned that for the preferred embodiments illustrated inFIGS. 1C to 1E, as in all of the following embodiments also, thedrawings show views of the polarization splitting plane only, as it isprimarily in this respect that these embodiments differ from that shownin FIGS. 1A. It is to be understood, however, that for each of thedrawings in FIGS. 1C to FIG. 7, there is also a corresponding wavelengthdispersion plane, similar to that of FIG. 1B, oriented at an angle,generally orthogonally, to the polarization splitting planes shown inFIGS. 1C to FIG. 7.

Reference is now made to FIGS. 2A and 2B, which schematically illustratea reflective wavelength selective optical switch, constructed andoperative according to a further preferred embodiment of the presentinvention. FIG. 2A is a view of the device from the polarization splitplane, while FIG. 2B is a view of the same device from the wavelengthdispersion plane. In the embodiment of FIGS. 2A and 2B, a reflectingsurface 60 is added after the liquid crystal device 20, so that thepolarized components of the beam are returned back along their incidentpath. This embodiment is thus similar in construction to the embodimentof FIGS. 1A and 1B, except that use is made of the symmetry of thedevice on either side of the polarization rotating component, in orderto simplify construction, and to reduce even further the number ofcomponents used. Though the reflecting surface 60 in the embodiment ofFIGS. 2A and 2B is shown as a separate device, according to a furtherembodiment, the reflecting surface can be applied to the back side ofthe liquid crystal device 20. The other components in the embodiment ofFIGS. 2A and 2B are labeled with the same characters as in FIGS. 1A and1B, and in general, for the incident signals, they have essentiallyidentical functions to those in FIGS. 1A and 1B. In addition, withrespect to the output signals, after passage through the liquid crystaldevice 20 and after reflection from the reflective surface 60 of FIGS.2A and 2B, these components have the equivalent functions to those ofthe output signals of FIGS. 1A and 1B, after transmission through theliquid crystal device 20 of FIGS. 1A and 1B. Thus, for example, thediffraction grating 16 is operative both to wavelength disperse theinput signals, and to wavelength combine the output signals. Likewise,the single imaging lens 18 both images the input signals from thegrating 16 onto the plane of the liquid crystal element 20, andconfocally images the output signals from the plane of the liquidcrystal element onto the grating 16.

The polarization beam splitter 14 of the input signals also acts as apolarization beam combiner for the output signals. However, since theinput and output signals both pass through this same component, unlikethe embodiment of FIGS. 1A and 1B, both input and output fibers must beconnected to this component, and a means provided for separating theinput from the output signals. This is preferably achieved by theaddition of circulators 54 and 56 at the ports of the polarization beamsplitter/combiner 14 in the embodiment shown in FIGS. 2A and 2B. Thecirculator 56 is not shown in FIG. 2B, as it lies perpendicular to theplane of the drawing, and beneath the polarization beamsplitter/combiner 14.

The signal having the wavelength which it is desired to be switched isinput from fiber 12 to port A of circulator 54. Since the direction ofpropagation of the circulator shown in the preferred embodiment of FIG.2A is anti-clockwise, the signal exits the circulator at the port Bfiber, is collimated by a lens at the end of the fiber and enters thepolarization beam splitter/combiner 14. For a particular wavelengthchannel, for instance λ₁ in the embodiment shown, if the pixel 24 forthat channel is not activated, the polarization of the components of thesignal are unchanged, but since the polarization component positions areswitched because of the imaging process onto the liquid crystal devicepixels, the signal returns after reflection and polarizationrecombination, to port B of circulator 56, which then outputs the signalvia port C to output fiber 58. If on the other hand, the pixel 24associated with the wavelength channel λ₁ is activated and generates aphase difference of π/2, the signal has its polarization componentsreversed, and consequently, after recombination in the polarization beamsplitter/combiner 14, is focused by the collimation lens to output toport B of circulator 54, and from there to port C and out to fiber 52.Thus, an input signal on fiber 12, having a wavelength λ₁, can beswitched between output fibers 58 and 52, according to the setting ofthe control voltage of pixel 24. In the same way, any other wavelengthwithin the range of the dispersive element 16 can be switched by meansof a control voltage applied to the appropriate pixel 24, 26, 28, . . .of the liquid crystal device 20.

According to another preferred embodiment of the present invention, asecond input fiber 50 is disposed at the input port A of the circulator56, and inputs its signal from port B to the polarization beamsplitter/recombiner 14. On return from its round trip through theswitch, the signal is directed either to fiber 52 if the relevant pixelis not activated, or to fiber 58 if the pixel is activated. Thisembodiment, like that of FIGS. 1A and 1B, can also therefore be used asa 2×2 optical switching network.

According to yet another preferred embodiment of the present invention,the input and output signals can be separated by using dual fibercollimators instead of the circulators shown in the embodiment of FIGS.2A and 2B. Such an embodiment is shown in FIG. 3, where only the inputsto the PBS are shown. The first channel of the 2×2 switching network hasa dual fiber collimator 62, with input fiber 66 and output fiber 67. Thesecond channel of the 2×2 switching network has a dual beam collimator64, with input fiber 68 and output fiber 69. Operation of the switch isotherwise similar to that using circulators, as shown in FIGS. 2A and2B.

In order to decrease the PDL of the reflective switches of FIGS. 2A and2B, according to a further preferred embodiment of the presentinvention, a quarter wave plate 23 may added in juxtaposition to thepolarization modulating element 24. Since the beam makes two traversesthrough this plate, the effect is that of a half wave plate 22, asdescribed in relation to FIGS. 1A and 1B. Alternatively and preferably,any other polarization rotating element which rotates the polarizationdirection by 90°, such as a Faraday rotator, can be used for thispurpose instead of a quarter wave plate.

Reference is now made to FIG. 4A, which is a schematic illustration of amultiple channel wavelength selective switch module, according to yetanother preferred embodiment of the present invention. The embodimentshown in FIG. 4 is similar to that shown in FIGS. 1A and 1B except thata pair of 2×2 switches are stacked one on top of the other, andpreferably utilize a common dispersive element 70 but separate focussinglenses, 72, 73 74, 75 and a common wavelength combining element 78. Eachswitch utilizes its own liquid crystal array 76, 77, in order to enableindependent operation for the two switches. Such an embodiment thusenables a more compact and component economic device to be constructedin a single package. Alternatively and preferably, a single liquidcrystal array may be used, as indicated by the dotted lines joining whatwould then be the two “parts” 76, 77 of the single element, with thepixels of separate rows being used to control the switching of eachstacked switch.

Reference is now made to FIG. 4B, which is a schematic illustration ofanother preferred embodiment of a wavelength selective switch module ofthe present invention. The switch array shown in FIG. 4B is similar tothat shown in FIG. 1A, or in half of the multiple channel embodiment ofFIG. 4A except that at each input and output, instead of a single fibercollimator, a multiple fiber collimator is used. In the preferredembodiments shown in FIG. 4B, triple fiber collimators 79 are shown,which are constructed by having three fiber in the same ferrule in frontof the collimating lens of the collimator. Such an embodiment enablesthe switch to work as a multiply parallel, wavelength selective switch,which is useful for providing switching capability with channelredundancy, as is known in the art. It is to be understood that althoughsuch a multiply parallel application has been illustrated for theswitching application shown in FIG. 4B, it can be utilized in any of theswitch applications shown in the other embodiments described in thisspecification, where such use is relevant, whether transmissive orreflective, and whether configured as a single switch, or as a stackedarray of switches.

Reference is now made to FIG. 5 which is a schematic illustration ofanother multiple channel wavelength selective switch, according to afurther preferred embodiment of the present invention. This switchdiffers from that shown in FIG. 4A in that common focussing lenses 80,82 are used, as well as a single liquid crystal array 84 which servesall of the wavelength channels in both separate switches. This allowseven greater economy of component use in the switch.

Stacking of switches, as shown in the transmissive embodiments of FIGS.4A, 4B and 5, can also be performed for the reflective wavelengthselective switch embodiments shown in FIGS. 2A and 2B. One preferredexample of such a stacked switch module is shown in FIG. 6, in whichsome of the operative elements of the switches, namely the grating 90and the reflective liquid crystal element 94, are common. In order tomaintain planar geometry at the reflective liquid crystal plane,separate focusing lenses 92, 93, are required. Individual polarizationbeam splitters 96, 98 are used, as in the transmissive embodiments, toinput and output the signals. In the preferred embodiment of thereflective switch array shown in FIG. 6, the input and output beams ofeach channel are separated by means of dual fiber collimators 95, asshown in FIG. 3, to illustrate the difference of this preferred aspectof these embodiments from that shown in FIGS. 2A and 2B wherecirculators are used. It is to be understood that such a stacked switchmodule can also be constructed with less of its components common,similar to the embodiment shown in FIG. 4A.

Reference is now made to FIG. 7 which is a schematic illustration ofanother reflective multiple channel, wavelength selective switch,according to a further preferred embodiment of the present invention.This switch differs from that shown in FIG. 6 in that the focussing lens102 too is common to both channels. In order to achieve correctreflective operation in this embodiment, the reflective surface 106associated with the liquid crystal element 104 is preferably constructedwith a concave profile, with a radius of curvature equal to the focallength of the lens 102, such that any beam focussed by the lens onto thereflective surface is returned along its incident path. In order tomaintain the correct angle of optical incidence for this to occurindependently for each channel, the PBS's 96, 98 operating as theincident and receiving units must preferably be aligned at an angle suchthat after refraction through the lens 102, the incident beams aredirected at the correct angle onto the concave reflective surface 106,and impinge thereon at spatially different positions. If the PBS's wereto be aligned axially, the beams would be focussed at essentially thesame points and could not be independently controlled by the liquidcrystal element 104.

In the preferred embodiment shown in FIG. 7, the grating 100 is shown asa transmissive grating, with the PBS's 96, 98, located in the positionsshown in FIG. 7. Alternatively and preferably, a reflective grating maybe used, in which case the PBS's are located such that the input andoutput beams of each channel are directed along the directions indicatedby the arrows 108, 109.

In the above-described embodiments of the present invention, the pixelsare described as being activated when a voltage is applied to them torotate the polarization direction, or un-activated when no voltage isapplied, and the polarization of traversing light is unaffected. Inpractice, this description is an idealized description since in general,even without the application of any drive voltage, there will be somepolarization rotation of the traversing light signals because of basicbirefringence of the liquid crystal material. In general therefore, theunactivated state is understood to mean that state obtained when apolarization rotation of 2nπ is obtained, where n is an integer notincluding zero, even if that state requires the application of a voltageto the element in question. The “voltage” required to activate thatelement is then the difference in voltage required between theunactivated and the activated state. Furthermore, the voltage requiredto switch an element may be a function of the wavelength of the lightbeing switched, and the switch controller is thus preferably programmedto supply the correct switching voltage to each pixel in accordance withthe wavelength of the light traversing that pixel.

In FIGS. 1A, 1B, 2A and 2B, the light beams are shown having a finitewidth only within the dispersive bounds of the switch, in order toillustrate the focusing effect through the liquid crystal element. Forthe passage through the entry and output components, including thePBS's, the beams are delineated by single lines only down the center ofthe beam, for reasons of clarity. It is thus not meant to be understoodfrom the drawings that the beam width changes in passage through thedispersive elements.

Although the above preferred embodiments of the wavelength selectiveswitch have been described using a liquid crystal element as thepolarization rotating element, it is understood that the invention isequally operable using any other suitable type of controlledpolarization rotating element known in the art. Likewise, although agrating has been used as the wavelength dispersing element, it isunderstood that the invention is equally operable using any other typeof wavelength dispersing element. Likewise, although fibers have beenshown to represent the input and output means for the optical signals,these being the most common medium for transferring optical information,it is understood that the invention is not meant to be limited to thistype of input and output means.

It is appreciated by persons skilled in the art that the presentinvention is not limited by what has been particularly shown anddescribed hereinabove. Rather the scope of the present inventionincludes both combinations and subcombinations of various featuresdescribed hereinabove as well as variations and modifications theretowhich would occur to a person of skill in the art upon reading the abovedescription and which are not in the prior art.

1. A wavelength dependent switch, comprising in sequence: a polarizationbeam splitter having at least a first input port, for receiving at leastone input beam having at least two wavelength components, saidpolarization beam splitter being operative to spatially split each ofsaid at least one input beam into a pair of beams of separatepolarization components, a first dispersive element receiving said pairof beams of separate polarization components, and operative to spatiallydisperse said wavelength components of each of said pair of beams ofseparate polarization components in a dispersion plane disposed at anangle to the plane in which said polarization components are split; afirst focusing element receiving said wavelength components of each ofsaid pair of beams of separate polarization components from said firstdispersion element, without any intervening polarization manipulationelement; a polarization modulating element disposed in the focal regionof said first focusing element, without any intervening polarizationdependent deflection element between said first focusing element andsaid polarization modulating element, said polarization modulatingelement being pixelated generally along the direction of the dispersionsuch that each pixel is associated with a separate wavelength component,each pixel of said polarization modulating element being operative tochange the state of the polarization of light passing through said pixelaccording to a control signal applied to said pixel; a second focusingelement receiving said light after passing through said polarizationmodulating element; a second dispersive element receiving light fromsaid second focusing element, and operative to combine said separatewavelength components of each of said pair of beams intomulti-wavelength beams; and a polarization beam combiner having twooutput ports, said polarization beam combiner receiving each of saidpair of multi-wavelength beams, and operative to combine said separatepolarization components such that each wavelength component of said atleast one input beam is directed to one of said two output portsaccording to the control signal applied to said pixel associated withsaid wavelength.
 2. A wavelength dependent switch according to claim 1,wherein said polarization modulating element is a liquid crystalelement.
 3. A wavelength dependent switch according to claim 1, whereinsaid angle is such that said dispersion plane is essentially orthogonalto said plane in which said polarization components are split.
 4. Awavelength dependent switch according to claim 1, wherein said separatepolarization components are two orthogonal components.
 5. A wavelengthdependent switch according to claim 1, wherein said polarization beamsplitter also comprises a second input port, such that said switch isoperative to switch a wavelength component of a signal input to eitherof said first and said second input ports, to either of said outputports, according to the control signal applied to said pixel associatedwith said wavelength component.
 6. A wavelength dependent switchaccording to claim 1 and also comprising a half wave plate disposed injuxtaposition to said polarization modulating element, operative toreduce polarization dependent losses in the switch.
 7. A wavelengthdependent switch according to claim 1, and also comprising a first halfwave plate disposed in the path of one of said pair of beams of separatepolarization components emerging from said polarizing beam spliffer; anda second half wave plate disposed in the path of another one of saidpair of beams of separate polarization components entering saidpolarizing beam combiner, such that polarization dependent losses in theswitch are reduced.
 8. A wavelength dependent switch according to claim1 and wherein at least one of said input beams is input to said switchby means of a multiple fiber collimator, such that said switch operatesas a multiply parallel wavelength selective switch.
 9. A wavelengthdependent switch module comprising a plurality of wavelength dependentswitches according to claim 1, and wherein at least two of saidwavelength dependent switches utilize a common one of at least one of adispersive element, a focusing element and a polarization modulatingelement.
 10. A wavelength dependent switch according to claim 1 andwherein at least one of said polarization beam splitter and saidpolarization beam combiner is of a split prism construction.
 11. Awavelength dependent switch according to claim 1 and wherein at leastone of said polarization beam splitter and said polarization beamcombiner is of birefringent crystal construction.
 12. A wavelengthdependent switch according to claim 1 and wherein said at least oneinput beam comprises one input beam, such that said switch operates as a1×2switch.
 13. A wavelength dependent switch according to claim 1 andwherein said at least one input beam comprises one input beam, and onlyone of said two output ports is used such that said switch operates as a1×1 switch.
 14. A wavelength dependent switch according to claim 1 andwherein said at least one input beam comprises two input beams, suchthat said switch operates as a 2×2switch.
 15. A wavelength dependentswitch, comprising in sequence: a polarization beam splitter having afirst and second port, at least one of said ports receiving an inputbeam comprising at least two wavelength components, said polarizationbeam splitter being operative to spatially split each of said at leastone input beam into a pair of beams of separate polarization components;a dispersive element receiving said pair of beams of separatepolarization components, and operative to spatially disperse saidwavelength components of each of said pair of beams of separatepolarization components in a dispersion plane disposed at an angle tothe plane in which said polarization components are split; a focusingelement receiving said wavelength components of each of said pair ofbeams of separate polarization components from said dispersion element,without any intervening polarization manipulation element; apolarization modulating element disposed in the focal region of saidfocusing element without any intervening polarization dependentdeflection element between said focusing element and said polarizationmodulating element, said polarization modulating element being pixelatedgenerally along the direction of the dispersion such that each pixel isassociated with a separate wavelength component, each pixel of saidpolarization modulating element being operative to change the state ofthe polarization of light passing through said pixel according to acontrol signal applied to said pixel; and a reflecting surface operativeto reflect said light back in sequence through said pixelatedpolarization modulating element, said focusing element, said dispersiveelement and said polarization beam splitter, such that each wavelengthcomponent of said light is output to one of said first and second portsaccording to the control signal applied to said pixel associated withsaid wavelength.
 16. A wavelength dependent switch according to claim15, wherein said polarization modulating element is a liquid crystalelement.
 17. A wavelength dependent switch according to claim 15,wherein said angle is such that said dispersion plane is essentiallyorthogonal to said plane in which said polarization components aresplit.
 18. A wavelength dependent switch according to claim 15, whereinsaid separate polarization components are two orthogonal components. 19.A wavelength dependent switch according to claim 15 and also comprisinga circulator disposed at at least one of said first and second ports,such that said input beam at said at least one of said first and secondports is separated from light output to said at least one of said firstand second ports.
 20. A wavelength dependent switch according to claim15 and also comprising a dual fiber collimator disposed at at least oneof said first and second ports, such that said input beam light at saidat least one of said first and second ports is separated from lightoutput to said at least one of said first and second ports.
 21. Awavelength dependent switch module comprising a plurality of wavelengthdependent switches according to claim 15 and wherein at least two ofsaid wavelength dependent switches utilize a common one of at least oneof a dispersive element, a focusing element and a polarizationmodulating element.
 22. A wavelength dependent switch according to claim15 and wherein said at least one input beam comprises one input beam,such that said switch operates as a 1×2 switch.
 23. A wavelengthdependent switch according to claim 15 and wherein said at least oneinput beam comprises one input beam, and only one of said first andsecond ports is used to output light, such that said switch operates asa 1×1 switch.
 24. A wavelength dependent switch according to claim 15and wherein said at least one input beam comprises two input beams, suchthat said switch operates as a 2×2 switch.
 25. A wavelength dependentswitch according to claim 15 and also comprising a quarter wave platedisposed in juxtaposition to said polarization modulating element,operative to reduce polarization dependent losses in the switch.
 26. Awavelength dependent switch, comprising in sequence: a polarization beamsplitter having at least a first input port, for receiving at least oneinput beam having at least two wavelength components, and operative tospatially split each of said at least one input beam into a pair ofbeams of separate polarization components, a first dispersive elementreceiving said pair of beams of separate polarization components, andoperative to spatially disperse said wavelength components of each ofsaid pair of beams of separate polarization components in a dispersionplane disposed at an angle to the plane in which said polarizationcomponents are split; a first focusing element receiving said wavelengthcomponents of each of said pair of beams of separate polarizationcomponents; a polarization modulating element disposed in the focalregion of said first focusing element, said polarization modulatingelement being pixelated generally along the direction of the dispersionsuch that each pixel is associated with a separate wavelength component,each pixel of said polarization modulating element being operative tochange the state of the polarization of light passing through said pixelaccording to a control signal applied to said pixel; a second focusingelement receiving said light after passing through said polarizationmodulating element; a second dispersive element receiving light fromsaid second focusing element, and operative to combine said separatewavelength components of each of said pair of beams intomulti-wavelength beams; and a polarization beam combiner having twooutput ports, said polarization beam combiner receiving each of saidpair of multi-wavelength beams, and operative to combine said separatepolarization components such that each wavelength component of said atleast one input beam is directed to one of said two output portsaccording to the control signal applied to said pixel associated withsaid wavelength; wherein said separate polarization components directedto a first one of said two output ports traverse essentially the sameoptical paths into said polarization beam combiner as those of saidseparate polarization components directed to a second one of said outputports.
 27. A wavelength dependent switch according to claim 26, whereinsaid polarization beam splitter also comprises a second input port, suchthat said switch is operative to switch a wavelength component of asignal input to either of said first and said second input ports, toeither of said output ports, according to the control signal applied tosaid pixel associated with said wavelength component, and wherein saidspatially split polarization components of said input beams from saidfirst input port traverse essentially the same optical paths on exitingsaid polarization beam splitter as those of said spatially splitpolarization components of said input beams from said second input port.28. A wavelength dependent switch, comprising in sequence: apolarization beam splitter having a first and second port, at least oneof said ports receiving an input beam comprising at least two wavelengthcomponents, and operative to spatially split each of said at least oneinput beam into a pair of beams of separate polarization components; adispersive element receiving said pair of beams of separate polarizationcomponents, and operative to spatially disperse said wavelengthcomponents of each of said pair of beams of separate polarizationcomponents in a dispersion plane disposed at an angle to the plane inwhich said polarization components are split; a focusing elementreceiving said wavelength components of each of said pair of beams ofseparate polarization components; a polarization modulating elementdisposed in the focal region of said focusing element, said polarizationmodulating element being pixelated generally along the direction of thedispersion such that each pixel is associated with a separate wavelengthcomponent, each pixel of said polarization modulating element beingoperative to change the state of the polarization of light passingthrough said pixel according to a control signal applied to said pixel;and a reflecting surface operative to reflect said light back insequence through said pixelated polarization modulating element, saidfocusing element, said dispersive element and said polarization beamsplitter, such that each wavelength component of said light is output toone of said first and second ports according to the control signalapplied to said pixel associated with said wavelength; wherein saidspatially split polarization components of said input beams from saidfirst input port traverse essentially the same optical paths on exitingsaid polarization beam splitter as those of said spatially splitpolarization components of said input beams from said second input port.29. A wavelength dependent switch, comprising in sequence: apolarization beam splitter having a first and second port, at least oneof said ports receiving an input beam comprising at least two wavelengthcomponents, said polarization beam splitter being operative to spatiallysplit each of said at least one input beam into a pair of beams ofseparate polarization components; a dispersive element receiving saidpair of beams of separate polarization components, and operative tospatially disperse said wavelength components of each of said pair ofbeams of separate polarization components in a dispersion plane disposedat an angle to the plane in which said polarization components aresplit; a focusing element receiving said wavelength components of eachof said pair of beams of separate polarization components from saidfirst dispersion element; a polarization modulating element disposed inthe focal region of said focusing element without any interveningpolarization dependent deflection element between said focusing elementand said polarization modulating element, said polarization modulatingelement being pixelated generally along the direction of the dispersionsuch that each pixel is associated with a separate wavelength component,each pixel of said polarization modulating element being operative tochange the state of the polarization of light passing through said pixelaccording to a control signal applied to said pixel; and a reflectingsurface operative to reflect said light back in sequence through saidpixelated polarization modulating element, said focusing element, saiddispersive element and said polarization beam splitter, such that eachwavelength component of said light is output to one of said first andsecond ports according to the control signal applied to said pixelassociated with said wavelength, wherein said spatially splitpolarization components of said input beam from said first input porttraverse essentially the same optical paths within said switch afterexiting said polarization beam splitter as those of said spatially splitpolarization components of said input beam from said second input port.